Multi-Shaft Vacuum Pump With Circular Translation Cycle

ABSTRACT

The invention relates to a vacuum pump comprising: a fixed disc ( 8 ) which is solidly connected to a fixed body ( 2 ) and which is equipped with a spiral-shaped element ( 14 ); a movable disc ( 4 ) facing the fixed disc ( 8 ) and having a spiral-shaped element ( 16 ) which is interleaved with the spiral ( 14 ) of the fixed disc ( 8 ); a connection between the fixed body ( 2 ) and the movable disc ( 4 ), which is provided with at least two cranks ( 18 ) each comprising two shafts ( 26, 28 ) that are displaced with the same eccentricity; and motor means ( 6 ) which are used to drive the movable disc ( 4 ) in a circulation translational motion and which comprise a motor ( 6 ) having an output shaft ( 30 ) which is parallel to the axes of the discs, said output shaft ( 30 ) of the motor ( 6 ) being centred on the axis of the movable disc ( 4 ) and driving same with the aid of a coupling piece ( 32 ) having the same eccentricity as the cranks ( 18 ).

The present invention concerns a multi-shaft vacuum pump with a circular translation cycle.

A pump of this type generally includes a fixed body with a fixed disk having a spiral-shaped projection on one of its faces and a mobile disk facing the fixed disk and having on the same side as the fixed disk a corresponding spiral-shaped projection. The two spirals, one fixed and one mobile, have the same angular amplitude and are interleaved with each other. A mechanism drives movement of the mobile disk in circular translation relative to the fixed disk. The connection between the mobile disk and the fixed body of the pump is provided by a plurality of shafts, usually three shafts.

Such a pump is described in the document FR-A-2 141 402, for example. As indicated in that document, such a pump can be a totally dry and sealed pump. It gives excellent results but has the drawback of comprising many parts and of being bulky. Note also that the pump described in the above document uses a cooling liquid to cool the parts.

An object of the present invention is therefore to provide a new, dry, sealed and clean vacuum pump (i.e. a vacuum pump with no risk of pollution of the pumped fluid) that is of simple structure and also compact. This pump should preferably discharge at atmospheric pressure the gaseous fluid aspirated to create the vacuum at atmospheric pressure. Furthermore, maintenance of this new pump should be facilitated compared to known prior art pumps.

To this end, the invention proposes a circular translation cycle vacuum pump including:

-   -   a fixed disk fastened to a fixed body and having a spiral-shaped         projection on one of its faces,     -   a mobile disk facing the fixed disk and having on the same side         as the fixed disk a spiral-shaped projection interleaved with         the spiral-shaped projection of the fixed disk,     -   a connection between the fixed body and the mobile disk         including at least two cranks, each crank including two parallel         shafts that are offset with the same eccentricity,     -   drive means for driving movement of the mobile disk in circular         translation relative to the fixed disk.

According to the invention, the drive means include a motor having an output shaft parallel to the axes of the fixed disk and the mobile disk, the output shaft of the motor is substantially centered on the axis of the mobile disk and drives the latter with the aid of a coupling part having an eccentricity corresponding to the eccentricity of the cranks, the two spiral-shaped projections have no contact with each other and have an axial clearance controlled by the mounting of the cranks, and the axial forces exerted on the pump are absorbed essentially by the cranks.

This new multi-shaft vacuum pump structure is highly beneficial because it improves the distribution of forces within the pump, especially in the various rolling bearings. Such a pump is clean because a pumped gaseous fluid passing through the pump does not come into contact with any part subject to wear, such as a seal or the like.

In a preferred embodiment of a vacuum pump according to the invention, the motor is fixed to a plate parallel to the mobile disk and this plate has housings for one shaft of each crank. In this way, the plate is disposed at the heart of the pump and carries both the motor and the mobile disk. On the one hand, this simplifies the structure of the pump and, on the other hand, makes it more compact. In this embodiment, the plate forms the end wall of a cylindrical tube in which the motor is housed, for example. The fixed body then includes an external casing connecting the cylindrical tube to the fixed disk, for example.

In a vacuum pump of the invention, a metal bellows can be used to connect the periphery of the mobile disk to the fixed body of the vacuum pump in a sealed manner. Such a bellows provides a seal between a part mobile in (circular) translation and a fixed body. If the vacuum pump includes a cylindrical tube, the metal bellows preferably surrounds the cylindrical tube. In this way, the bellows can be of relatively large size and the stresses exerted on it are therefore lower than for a small bellows.

The invention advantageously provides for a second metal bellows, also mounted in sealed manner between the periphery of the mobile disk and the fixed body, to surround the first metal bellows. This produces a space between the two metal bellows. By monitoring the pressure in this space, it is possible to detect failure of either bellows immediately. As a result, it is possible to ensure perfect control over the sealing of the pump.

In the embodiment in which the vacuum pump includes a cylindrical tube, there may be provision for the cylindrical tube to receive on the side opposite the plate a fan and for orifices to be provided in the plate to enable ventilation of the housing of the motor.

The vacuum pump can also include ventilation means for cooling the face of the fixed disk opposite the mobile disk.

When the pump is equipped with such ventilation means, it is not necessary to provide a cooling liquid circuit to prevent overheating of the pump in operation. This avoids one possible source of pollution of the gaseous fluid pumped by the pump of the invention.

For even better cooling of the disks of the pump, there may be provision for at least one projection forming a spiral on one disk to be perforated in the vicinity of the heart of the spiral.

For discharging the pumped gaseous fluid at atmospheric pressure the fixed disk has, for example, substantially at the center of the spiral, a discharge orifice discharging onto the face opposite the spiral and this discharge orifice can be provided with a discharge valve.

To encourage discharging of the pumped fluid at atmospheric pressure, the thickness of the projections forming a spiral has at the heart of the spiral a thickness increasing to form a bulb-shaped end of the spiral.

Details and advantages of the present invention will emerge more clearly from the following description, given with reference to the appended diagrammatic drawings, in which:

FIG. 1 is a view in longitudinal section of a vacuum pump of the invention,

FIG. 2 is a view in section taken along the section line II-II in FIG. 1, and

FIG. 3 is a view in section taken along the section line III-III in FIG. 1.

There is seen in FIG. 1 a circular translation cycle vacuum pump. That pump includes in particular a fixed body 2, a mobile disk 4 and a motor 6.

In the preferred embodiment represented in FIG. 1, the fixed body 2 consists essentially of three elements: a fixed disk 8, a casing 10 and a cylindrical tube 12.

The fixed disk 8 has on its interior face a spiral-shaped projection 14. The shape of the latter can be seen in FIG. 2. The mobile disk 4 faces the fixed disk 8. On its face oriented toward the fixed disk 8, the mobile disk also has a spiral-shaped projection 16. The spiral 16 of the mobile disk 4 is interleaved with the spiral 14 of the fixed disk 8, as can also be seen in FIG. 2 (to simplify the view, FIG. 3 represents neither the fixed spiral 14 nor the casing 10). The spirals 14 and 16 are separated from each other by a constant axial clearance, regardless of the position of the mobile disk 4 relative to the fixed disk 8. This clearance is of the order of 5/100 mm, for example. There is also a radial clearance between the spirals. That radial clearance is of the same order of magnitude as the axial clearance (approximately 0.05 mm).

The fixed body 2 supports the mobile disk 4 through the intermediary of three cranks 18 of equal eccentricity E that are coupled and synchronized to obtain a movement of the mobile disk 4 in circular translation relative to the fixed body 2, and more particularly its fixed disk 8, when the pump is operating.

The cranks 18 connect the mobile disk 4 to the fixed disk 8 so that, on the one hand, the mobile disk 4 is perfectly guided to obtain virtually perfect control over the axial clearance at the spiral-shaped projections 14, 16 and, on the other hand, the axial forces between these two disks are absorbed by these cranks. The axial clearance referred to here in respect of the spiral-shape projections 14, 16 indicates that between the summit of one spiral 14, 16 and the facing disk 4, 8 there is a substantially constant clearance 19 (of the order of 0.05 mm).

The cylindrical tube 12 has a body having longitudinal axis is parallel to the axis of the fixed disk 8 and the mobile disk 4 and a transverse end wall 20 which is therefore situated in a plane parallel to those of the disks. This end wall 20 includes three bores 22 each receiving a crank 18. Each crank 18 includes a journal 24, a lower shaft 26 and an upper shaft 28. Each journal 24 is situated at the interface between the end wall 20 and the mobile disk 4. The lower shaft 26 of each crank 18 is housed with corresponding bearings in a bore 22 in the end wall 20 while each of the upper shafts 28 is housed with corresponding bearings in housings provided for this purpose in the mobile disk 4. The axes of the lower shafts 26 and the upper shafts 28 are parallel and offset by a distance corresponding to the eccentricity E.

Each bore 22 has a shoulder against which rest ball-bearings guiding the lower shaft 26 of a crank 18. The journal 24 rests on these ball-bearings on the side opposite the aforementioned shoulder. A similar arrangement is used for the housings provided in the mobile disk 4. Each of those housings has a shoulder against which abut ball-bearings receiving the upper shaft 28 of the crank 18 and, on the side opposite that shoulder, the journal 24 rests on the ball-bearing. These bearings are retained in their housings in the mobile disk 4 and the end wall 20, with the aid of a ring retained by screws 29.

This type of assembly corresponds to a vacuum pump in which the mobile disk 4 tends to move axially toward the fixed disk 8.

The movement of the mobile disk 4 in circular translation relative to the fixed disk 8 is driven by the motor 6. The latter is disposed inside the cylindrical tube 12 and fixed to its end wall 20. In the preferred embodiment of the invention shown in the drawings, the output shaft 30 of the motor 6 is connected to the mobile disk 4 by a coupling part 32. Here the latter takes the form of a sleeve including two aligned cylindrical circular housings offset by a distance corresponding to the eccentricity E. The cylindrical housing on the same side as the motor receives the output shaft 30 of the motor and the cylindrical housing on the same side as the mobile disk 4 receives a crank-pin 34 projecting from the face of the mobile disk 4 opposite the spiral 16 and a needle roller bearing. In this embodiment represented in the figures, the crank-pin 34 is centered relative to the mobile disk 4. The needle roller bearing essentially absorbs radial forces on the motor shaft. Note in the figure a ball-bearing in the vicinity of the output shaft 30. The only function of this bearing is to retain the motor shaft if the motor is demounted for maintenance or repair or changed.

Note in FIGS. 1 and 3 the presence of a balancing mass 36 around the coupling part 32 at the level of the crank-pin 34. This mass is calculated and positioned to provide dynamic balancing of the pump when the latter is operating.

In the embodiment that can be seen in FIG. 1, the casing 10 of the fixed body connects the periphery of the fixed disk 8 with the free edge of the cylindrical tube 12 at the end opposite the end wall 20.

Note also the presence of two metal bellows 38, 38′ in FIG. 1. Each of these bellows 38, 38′ is mounted on and sealed, on the one hand, to a peripheral rim of the mobile disk 4 and, on the other hand, to the casing 10. The bellows 38, 38′ are fixed by welding them, for example.

The two bellows 38, 38′ therefore surround the (fixed) cylindrical tube 12 inside which the motor 6 is located, the bellows 38′ surrounding the bellows 38. The presence of a single bellows is sufficient to achieve good operation of the vacuum pump of the invention. The second bellows is provided here for safety reasons. A pressure sensor connected to the empty space between the two bellows immediately detects the slightest leak in either of the bellows, the other bellows then still ensuring sealing of the pump.

The metal bellows 38, 38′ are used to render the pump fully sealed. They are protected against twisting caused by the kinematics of the drive system. To limit the stresses exerted on these bellows, they extend substantially the full height of the pump, as can be seen in FIG. 1. Shorter bellows (or a single bellows) might be equally suitable.

The casing 10 includes an inlet opening 42 which, in the embodiment shown, provides a radial inlet for the gaseous fluid in the vicinity of the fixed disk 8 and the mobile disk 4. An enclosure to be evacuated (not represented in the drawings) is connected to the inlet opening 42 of the pump (by means that are not shown). As soon as the pump is started, the gaseous fluid to be pumped is subjected to the continuous and progressive effect of compression obtained by the relative movement between the two spirals 14 and 16. In a manner that is known in the art (cf. for example FR-2 141 402), the gaseous fluid in the space between the exterior bellows 38′ and the casing 10 is driven towards a discharge orifice 44 at the center of the spiral 14 of the fixed disk 8. This discharge orifice 44 is fitted with a valve 46 for discharging the gaseous fluid at atmospheric pressure.

The spirals tend to become heated during operation. A first ventilation unit 48, represented very diagrammatically in FIG. 1, is disposed on the face of the fixed disk 8 opposite the face carrying the spiral 14. An arrow 50 represents one example of the movement of air for ventilating the fixed disk 8.

A second ventilation set 52 is disposed in the cylindrical tube 12 to cool the motor 6 and the mobile disk 4. This second ventilation unit 52, shown very diagrammatically, aspirates air from outside the pump to feed it into the latter along a path illustrated by way of example by arrows 54. To allow such flow of air, orifices are provided in the end wall 20 of the cylindrical tube and in the fixed body 2, to enable external air to penetrate between the exterior surface of the cylindrical tube 12 and the interior of the inner bellows 38.

To promote further the cooling of the fixed disk 8 and the mobile disk 4, a void 56 is provided in each of the spirals 14 and 16, as can be seen in the drawings. The spirals 14 and 16 each have at their end that is substantially at the center of the corresponding disk a bulbous portion 57 produced by enlarging the projection forming the corresponding spiral toward the center of the disk. Thus air set in motion by the ventilation units can enter the voids 56 and thereby improve the cooling of the spirals 14 and 16 and therefore of the corresponding disks 8 and 4.

The vacuum pump described hereinabove is a totally dry pump, which operates with no oil and no cooling liquid. This pump is also entirely sealed with perfect control of the seal, thanks in particular to the two bellows.

The pump described here is a so-called clean pump. The pumped fluid does not encounter any parts subject to wear (seals, etc.) from the inlet orifice 42 to the outlet orifice 44. The control of the axial clearance between the spiral-shaped projections 14, 16 means that the seals usually found between each spiral and the disk facing said spiral can be dispensed with. The absence of parts subject to wear along the whole of the path of the pumped fluid protects the latter from pollution.

A pump such as that described hereinabove can be used for relatively low flow rates. By way of purely illustrative and nonlimiting example, such a pump can generate a volume of the order of 30 m³/h, for example.

A vacuum pump according to the invention can find applications in the nuclear field, for example, in the fabrication of electronic components, in the pharmaceutical industry, in the foodstuffs industry, etc. This list of applications is not exhaustive, of course. This type of pump can be used anywhere great care is necessary to avoid any pollution of a transported fluid.

Although for prior art pumps of equivalent size it is usually necessary to provide coupling to a primary pump, the pump described can discharge directly at atmospheric pressure. This direct discharge at atmospheric pressure can be obtained here thanks in particular to the presence of a valve 46 at the outlet but also because of the bulbous portion 57 at the ends of the spirals. The good ventilation of the pump also encouraged by the voids 56 in the region of said bulbous portions is also important in enabling such rejection. In fact, given the variation of pressure, the heating effect in the pump is relatively strong and appropriate cooling must be provided.

The structure of the pump described is simple, even if it includes four shafts. Moreover, it produces a compact pump. Furthermore, maintenance of this pump is simplified. The number of seals and parts subject to wear is very small.

An important advantage of the structure described hereinabove with a central drive shaft and peripheral shafts is how it absorbs radial forces. These are absorbed essentially by the needle roller bearing described. The load on the bearings of the peripheral shafts is limited and this allows the use of bearings with a higher axial stiffness than the bearings used in comparable prior art pumps.

The present invention is not limited to the preferred embodiment described hereinabove by way of nonlimiting example. It relates equally to all variants that might occur to the person skilled in the art within the scope of the following claims.

For example, the number of shafts (cranks) can be modified without departing from the scope of the invention. Other modifications, concerning for example the shape of the fixed body, the cooling of the parts (possibly even using a cooling liquid), the mounting of the motor, the bellows, etc. can equally be made without departing from the scope of the invention. 

1-12. (canceled)
 13. Circular translation cycle vacuum pump including: a fixed disk (8) fastened to a fixed body (2) and having a spiral-shaped projection (14) on one of its faces, a mobile disk (4) facing the fixed disk (8) and having on the same side as the fixed disk (8) a spiral-shaped projection (16) interleaved with the spiral-shaped projection (14) of the fixed disk (8), a connection between the fixed body (2) and the mobile disk (4) including at least two cranks (18), each crank (18) including two parallel shafts (26, 28) that are offset with the same eccentricity, drive means (6) for driving movement of the mobile disk (4) in circular translation relative to the fixed disk (8), these drive means including a motor (6) having an output shaft (30) parallel to the axes of the fixed disk (8) and the mobile disk (4), and the output shaft (30) of the motor (6) being substantially centered on the axis of the mobile disk (4) and driving the latter with the aid of a coupling part (32) having an eccentricity corresponding to the eccentricity of the cranks (18), the two spiral-shaped projections (14, 16) having no contact with each other, characterized in that the two spiral-shaped projections (14, 16) have an axial clearance controlled by the mounting of the cranks (18), in that the axial forces exerted on the pump are absorbed essentially by the cranks (18), in that the motor (6) is fixed to a plate (20) parallel to the mobile disk (4), and in that this plate (20) has housings (22) for one shaft (26) of each crank (18).
 14. Vacuum pump according to claim 13, characterized in that the plate forms the end wall (20) of a cylindrical tube (12) in which the motor (6) is housed.
 15. Vacuum pump according to claim 14, characterized in that the fixed body (2) includes an external casing (10) connecting the cylindrical tube (12) to the fixed disk (8).
 16. Vacuum pump according to claim 14, characterized in that a metal bellows (38) connects the periphery of the mobile disk (4) to the fixed body (2) of the vacuum pump in a sealed manner.
 17. Vacuum pump according to claim 16, characterized in that the metal bellows (38) surrounds the cylindrical tube (12).
 18. Vacuum pump according to claim 16, characterized in that a second metal bellows (38′), also mounted in sealed manner between the periphery of the mobile disk (4) and the fixed body (2), surrounds the first metal bellows (38).
 19. Vacuum pump according to claim 14, characterized in that on the side opposite the plate (20) the cylindrical tube (12) receives a ventilation unit (52) and in that orifices are provided in the plate (20) to enable ventilation of the housing of the motor (6).
 20. Vacuum pump according to claim 13, characterized in that ventilation means (48) are provided for cooling the face of the fixed disk (8) opposite the mobile disk (4).
 21. Vacuum pump according to claim 13, characterized in that the fixed disk (8) has, substantially at the center of the spiral (14), a discharge orifice (44) discharging onto the face opposite the spiral (14) and in that this discharge orifice (44) is provided with a discharge valve (46).
 22. Vacuum pump according to claim 13, characterized in that at least one projection forming a spiral (14, 16) on a disk (8, 4) is perforated in the vicinity of the heart of the spiral (14, 16).
 23. Vacuum pump according to claim 13, characterized in that the thickness of the projections forming a spiral (14, 16) has at the heart of the spiral (14, 16) a thickness increasing to form a bulb-shaped end (57) of the spiral.
 24. Vacuum pump according to claim 13, characterized in that a metal bellows (38) connects the periphery of the mobile disk (4) to the fixed body (2) of the vacuum pump in a sealed manner.
 25. Vacuum pump according to claim 17, characterized in that a second metal bellows (38′), also mounted in sealed manner between the periphery of the mobile disk (4) and the fixed body (2), surrounds the first metal bellows (38). 